The invention relates to a method for operating an internal combustion engine, in particular a gas internal combustion engine, having an intake section and an engine with a number of cylinders. The invention also relates to a closed-loop control system for the internal combustion engine, and to an internal combustion engine. The internal combustion engine has, in the intake section, a supercharging system and a bypass for bypassing the supercharging system.
It is known generally to use throttle elements for load closed-loop control in internal combustion engines. Internal combustion engines are increasingly subjected to transient, that is to say in a most general way non-steady-state, operating modes even in the case of non-mobile applications, with the result that the load closed-loop control assumes increasing importance.
This proves comparatively complex, in particular, in gas internal combustion engines. Application fields of gas internal combustion engines are mobile applications, such as, for example, in the field of shipping or in the field of utility vehicles and heavy-duty motor vehicles, as well as stationary applications such as in block power stations, which are to be advantageously configured, in particular, for a fluctuating gas supply.
U.S. Pat. No. 6,131,552 generally discloses a fuel control system which can regulate the supply of gas to a mixture chamber as a function of a measured operating state of the engine. The control method in U.S. Pat. No. 6,131,552 A or other only load-dependent gas-metering systems also proves inadequate in the case of complex regulating systems.
Volumetric efficiency is usually a measure of the gaseous fresh charge of charge air which is supplied to an internal combustion engine in a charge mixture, wherein the volumetric efficiency also permits conclusions to be drawn about the quality of the intake system and of the intake process. The actual volumetric efficiency generally represents the ratio of the mass of fresh air in a charge mixture, which is actually fed to the engine or cylinder thereof during a working cycle. This real mixture mass is determined, with respect to the theoretical fresh charge mass, from the geometric swept volume and the theoretical charge density in the environmental state (in the case of freely aspirating engines), and in the case of supercharged engines the state of the fresh charge downstream of the compressor or downstream of the charge air cooler is taken into account here.
A series of factors, for example, the valve control times or the opening cross section of the valves, influence the fresh charge which is fed to a cylinder. Generally, these factors can be determined from a module for determining the supercharging of the engine to which an intake section model is stored. However, the fresh charge fed to the engine in a charge mixture actually corresponds to the theoretical only in exceptional cases. The volumetric efficiency is not a constant number for an engine but instead is dependent to a large degree on the rotational speed and the geometric conditions of an intake section and a combustion chamber which are present; to overcome this dependence it is possible to take into account, for example, a suitable characteristic diagram.
Intake section models are, in turn, known in engine regulators, basically in general internal combustion engines such as, for example, from EP 1 398 490 A2. These engines have in common the fact that by modeling the intake section—in the simplest case as a homogeneous pressure vessel in order to sense the dynamic processes in the air path—as a widespread basic concept, the storage behavior of the intake section (also referred to as intake manifold) is modeled by means of the filling and emptying method. In this context, the intake manifold is treated as a pressure vessel which is continuously filled with air through a throttle valve and from which the engine sucks air via the inlet valve through its intake behavior corresponding to the working cycle.
However, it becomes apparent that a fuel supply of a gas internal combustion engine, in particular in the transient operating range of the internal combustion engine and with variable fuel qualities is much more complex. In addition, in the case of gas internal combustion engines it becomes apparent, in particular with respect to the formation of a spark-ignition gas internal combustion engine, that generally a load regulation, in particular the operation in the low load range and/or in the transient load range, can be problematic. Therefore, generally in the case of internal combustion engines, but in particular in the case of gas internal combustion engines, in the case of load shedding and when the throttle valves close, compressor pumping can occur when the compressor of the exhaust gas turbocharger is operated briefly outside its operating range as a result of a rapid reduction in the gas mass flow which is being fed by it. Furthermore, a load cut-in potential of a gas engine is very limited if said potential is opened in the steady-state mode.
At the same time, the intake section between the compressor outlet and the combustion chamber inlet is composed, in some cases, of large volumes which, as a result, store or output significant mixture masses. This applies, in particular, when pressure changes and/or temperature changes occur in the individual partial volumes when there are changes in load and/or rotational speed of the engine. As a consequence of mixture mass formation in a gas internal combustion engine which is, however, adapted only to a restricted degree to the operating point, and which is inprecise owing to the, in some cases, large volumes, high hydrocarbon emissions (HC emissions) or other increased emissions (NOx, CO or particles etc.) as well as poor efficiency levels as a result of unburnt combustion gas are to be expected.
During operation of gas engines, the mixture formation usually takes place upstream of the compressor of the exhaust gas turbocharging system; the present problem relates, in respect of the gas engine, not only to gas engines with central mixture formation but also to those with cylinder-specific mixture formation.
It is desirable to make operation of an internal combustion engine, in particular gas operation of a gas internal combustion engine, more advantageous, in particular in the transient, preferably low-load range, in accordance with the load requirements and emission conditions.